Mine face traversing mining head



Jan. 1, 1957 A. R. ANDERSON MINE FACE TRAVERSING MINING HEAD Filed May 5, 1951 7 Sheets-Sheet l fmvENTO/Qi Aavno Rfinpclesom, BY

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. 2'6 AEVID RAND ESON, Eiq. 8 BY HTT'Y Jan. 1, 1957 A. R. ANDERSON 2,775,126

MINE FACE TRAVERSING MINING HEAD Filed May 5, 1951 7 SheetsSheet 7 1/7 VENTO/Q Rig. Arevn 1:) RJANDEESON) HT?" 'x United States PatentO MINE FACE TIRAVERSINGHEA-D Arvid R. Anderson, Upper Arlington, hio,@assignor, by

mesne assignments, toiThel Golmolxflompany, azcorporationof Ohio Applicafion'May 5,?1 951, SerialNo."224;788

*4 .Claims. -w--(Cl..t262 9) This invention relates to a continuousmining machine which is adapted to remove coaliffromthelface of amine room.

An object ofthe' invention, thereforegisto, provide. an

improved mining machine ofithe above general type.

Anotherobject'ofthe' invention is v to provi'de animproved mining machine. ofthe .above general typeby which' coal is broken, out, .torn, and-.otherwiseremoved from a mine roomfaceby miningmechanism which is moved across the mineroom face whilethe mechanism breaks, cuts,'tears* and otherwise removes coal'ltherfrom.

Another object'of'the invention .is to .provide. an improved mining machine'as set forth inItheforegoing-objects wherein a coalmining mechanism adapted to break,

out, tear,or otherwise remove coal from a mineroom face is so disposed and moved" as toitravel continuously,

i-but L in difierent directions, over the mine :room face as it is operated toremove coalthere'from.

In carrying out theforegoingpobjects it is still another object of the invention'to'provide'an improved.,mining machine in -v'vhich the movement 'ofthe mining mechanism, while it travels across" or overi' the'rnine roomface to mine coal, --is continuous-but the "direction of which is changed automatically'whenever themining mechanism reachesits limit of travel -inany direction.

l Other objects ofthe invention-w-ill appear hereinafter, the novel features and-combinations being-setforth in the appended claims.

head section of the machine shown in Figs. 1- and'*2, 3

and 4, and 5;

Fig. 7- is a view in-sect-iom on a largersea-1e throughthe head section of the machineshown in Figa 'd the section being taken on 1ine"15- 'thereof;

Fig. 8 is a view in section, the' section being taken on line 1616 of- Fig. 7;

.Fig. 9 is a diagrammaticshowing of a hydraulic system employed in the machine shown -in combined Figs. 1 and 2, 3 and 4, and Figw5; and

Fig. 10 is .a diagrammatic view illustrating the-path of travel of the coalbreakingiand 'chip'ping' head and the relative positions of .certain control =valves with respect thereto.

In the accompanying .drawings :there ;are shown ttwo embodiments [of the: combination; coal tcutting and. loading machine, each of. which includes" asbasic loaderzwhich, -.unless otherwise 1 described, ris'iidentic'al iin construction and operation to a the apparatus 'idescribed .-in the-.applicaice 2 tion of Sterling 10.. Moon, SerialNo. 144,127, owned by the as'signee of the presentinventiomffiled' February'14, "1950, nowabandoned, for 21 Continuous Miningand LoadingMachine.

Referring to the embddiment'offithe machine shown in Figs. 1 through 10v 'ofithe "drawings, the machine includes .a truck portion having a'main frameifthat is supported, propelled forwardly or'rearwardly and 'steered by'aj pair of crawler devices 21,"one of which is positionedat each side of the imain frame 20. ".The .center'po'rtion "of the main' frame 20 forms a: machinery compartment .22. in which there is housed ahydraulic pump, gearing,clutches and the 'like (not'shown) that are:drivenby. an electric "motor 23 carried upon the'rear end'of themain'. frame20.

Adjacent its forward end, the machineisprovided with "what is essentially a pair 'of spaced :apartioaderbars 24 and '25 whickhave the "double function of tearing and "gathering coal from the mine floor and "conveying the loose coal rearwardly to the receivingend-ofa receiving 20 *or discharge conveyer designated -bythe rferencechar- "acter' 26 in'Fig. 3 .of-the drawings.

*T-he receiving' or discharge conveyer"26"extends over "the top of themachinery' compartment 22 of f thet'main frame 20 and the electric motor'2-3 through what is essen- 25 "tially atwo-section"trough that is 'formed" by a'rear dis- -charge-t-rough form-ing*boom27(Figs. 1-'-and"3), 'an'intermediatesection'28-and a 'hopper section 29. The dis- 3 "charge conveyer 26' is d'riven'at its frontend by" a head or cross shaft and sprocket, not shown, carried in the front end of main frame' 20 and it -is 'carried' atthe'rear end ofthe 'rear' discharge trough or boom 27 ""by-an -idler or t-ail'sha'ft and sprocket -whichalso isnot'shown. The return run of I the endless "conveyer 26 extends from the tail shaft and-'sprocketthrou'g'h the bottomportions of the conveyeri trough sections-to the head shaft.

The rear portion of the intermediate section 28 ofthe two-section 1 trough is pivoted J with respect to the 1 main 'frame 20 about a transverse horizont-al -aXis of a pair' of spaced 'pivot pins, one of which is indicated at "'30 1in' Fig. '=*1 of the drawings. In addition, the conveyor boom '27 -is pivoted about the vertical 'or upright axis indicated at 31 in-Fig. l-ofa turnt'able 32 that interconnects the front end of boom 27 and-the rear end 'of the-intermediate trough' section 28.

Horizontal swinging movement of boom 27 about the uprightaxis lila of turntable32 is provided by a pair of hydraulic pistonsmotors, 'one 'of which-is seen at '33 in Fig. 3 thatwactathroughnablesr35. .Motors 33 are 'each vtcontainediunder a guard shield s34. Verticalipivotal svvingningimovementt of the cboomt27 :and the-intermediatetconveyer trough section 28 about theihorizontalaaxis provided by pivot pins i30::is ieffected-fzbyaihydraulic motor, not :shown, that is' attachediatiits upper :end itor'a bracket .36

(see: Fig. l) rand. at itsibottom tozthe :main: frame 20.

Thenloaderibars 24xand. 25 ":havestheir. rear endsazconmected .to form auunitrby thexfrontkend of'mainfframe 1:20;.and2' they slope tupwariily thereon sand terminate over the front or -receiving1end of thereceiVing zort discharge :.conveyer 26. lThezbottoms of .the 1 loader 'barsl z lland 25 60. yore; spaced-above thezmain framei2fl sso thataa coal. eonveying trough or way 137 isiformedtbetween the two 'bars .24'and 25, and themmainframefio forms vthei'bottom of said. trough or .away at: its rear :portion. W.hile the 'rear ciportion .of eachsbar 7 24.:andi 225.:is stationary with respect :to the main frames-20,ethezprincipat portion: of 1 each of said bars, including the centrally upwardlyzisloping portioniand the front "generally'horizontal':portion, is pivotally :mounted as:. a. .unitiwabout a transverse rhorizontaliaxis so that the centralci and \forward 1 portions of -said: bars 124 iand-;25 a can beiadjusted; about said shorizontal axisp thus sadjusting the position of them twithrespect to the mine Pivotal mounting of said central and forward portions of the bars 24 and 25 is provided by spaced pivot joints between a common bottom plate frame member 38 (see Fig. 2) which rigidly and permanently connects the central portion of the bars 24 and 25. The conveyer bottom plate 38 is provided with pivot arms, one of which is indicated at 39, that are pivoted to the front end of main frame 20 by pivot joints. In other words, each of the bars 24 and 25 is provided with a rear portion which is rigidly connected to the main frame 20, while the central and forward portions thereof are pivotally mounted as a unit for adjustment about a transverse horizontal axis, thereby permitting the adjustment thereof with respect to said main frame 20 and the mine floor. This adjustment is accomplished by a hydraulic motor, not herein shown but which is shown and fully described in the above-identified application, Serial No. 144,127 of Sterling C. Moon.

Bars 24 and 25 carry endless gathering and loading chains 200 and 201, respectively, that are of similar structure but they travel in opposite directions, the former clockwise, the latter counterclockwise.

The chains 200 and 201 are provided with removable bits or picks 44 adapted to tear coal and gather it from the mine floor. These picks or bits 44 also assist in conveying the coal up the trough or way 37 from which it is spilled into the hopper trough section 29 and upon the front end of the receiving or discharge conveyer 26. It is, of course, evident, particularly by reference to Fig. 2 of the drawings, that the trough or way 37 is bottomless adjacent the forward portion wherein the bars 24 and 25 are substantially horizontal. Throughout this forward substantially horizontal portion of the bars 24 and 25 the mine floor forms the effective bottom of the trough or way 37 through which the coal is conveyed until it reaches the common bottom plate frame member 38 over which it travels to the main frame 20 and from the latter it is spilled upon the receiving or discharge conveyer 26.

The endless chains 200 and 201 are driven by sprockets 45 and 46, respectively, mounted above the machinery compartment 22 of the main frame 20. Because a portion of the bars 24 and 25 i swingable with respect to another portion and because the bars are curved upwardly substantially at the center of their lengths, the chains 200 and 201 are of the universal type.

It may appear that when the machine is advanced forwardly rectilinearly into coal on the mine floor that there might be left standing a thin rib or pile of compact coal that would enter the way 37 between the chains 200 and 201. However, in practice it has been found that vibrations of the machine and the lumps of coal being gathered and/ or broken from the mine floor prevent the formation of such a rib of coal.

It may be mentioned here that the loader chains 200 and 201 are particularly adapted to gather coal from the mine room floor and that they will perform some digging and trimming particularly on coal which has been partially relieved, such as scallops of coal which may appear on the mine bottom as a result of the action of the breaking and chipping arms as hereinbefore described. The loader chains 200 and 201 are not adapted to cut a deep kerf in solid coal but they are adapted to remove a thin layer of relieved coal from the floor.

It is to be understood that it is within the scope of this invention to substitute for the loader chains 200 and 201 combination kerf cutting chains, such as are shown in Figs. 2, 3 and 4, that are capable of cutting deep kerfs in solid coal.

The supplemental mining mechanism 202 includes a vertically adjustable frame 203 mounted for vertical rectilinear adjustment on a pair of laterally spaced guide posts 204 attached to the main frame 20, the frame 203 being provided with a pair of sliding guideways 205 slidable up and down along the guide posts 204. Power operated means, including a pair of screws 206, one associated with a nut carried on a boss 207 of each guideway 205, and motors which rotate the screws 206 effect raising and lowering of the frame 203. The screws 206 are held in fixed position by upper and lower brackets 208, 209 rigidly attached to posts 204. The screws 206 may be reversely operated or locked in adjusted position by means of individual rotary hydraulic motors 210 which drive them through gear trains contained in gear housings 211.

As clearly illustrated in Fig. 2 of the drawings, the frame 203 slopes upwardly and forwardly and at its forward end is provided with a generally laterally horizontal upright transversely or laterally extending track or guideway 212 which supports a breaking and chipping head 213 for lateral adjustment relative thereto and to the rest of the machine, including the main frame 20 and bars 24 and 25.

The head 213 includes a gear box or frame 214 which includes a plurality of intermeshing gears driving a pair of lower forwardly generally horizontal longitudinally extending shafts 215, 216 and a similar but shorter pair of upper shafts 217, 218. All of the shafts 215 to 218 are geared together, as clearly illustrated in Figs. 7 and 8 of the drawings and are driven from a single rotary hydraulic motor 219 (see Figs. 6 and 7) and drive gearing in a gear housing 220.

The lower shafts 215 and 216 carry breaker arms 221 and 222, respectively, which preferably follow the construction of the aforedescribed breaker arms 115118. The upper shafts 217 and 218 drive breaker arms 223 and 224, respectively, preferably similar in construction to the aforedescribed breaker arms 110113.

Mounting of the breaker arms 221 and 222 on the gear box 214 follows quite closely the mounting of the breaker arms 115 to 118 on the gear box 52, and the mounting of the breaker arms 223 and 224 on the gear box 214 follows closely the mounting of the breaker arms 110 to 113 on the gear box 52.

Breaker arms 221 and 223 are provided with a pair of v in-phase eccentrics 225 with which a trimmer plate 226 time.

is associated, carrying vertical or side and top horizontal bits 227 adapted to trim the scallops from the sides or ribs and tops or roof of the room, tunnel or entry which will be produced by the rotating breaker arms. The breaker arms 222 and 224 are similarly provided with eccentrics 228 and an associated trimmer plate 229 having vertical or side and top horizontal bits 230.

The direction of rotation of the breaker arms 221, 222, 223, and 224 is clearly illustrated in Fig. 5 of the drawings. The path of rotation of each overlaps the path of rotation of the two adjacent ones. The upper pair is, however, rearwardly of the lower pair, as a consequence of which there can be no interference between them. The upper pair are timed relative to each other to prevent interference, as are the lower pair. The lower pair of breaker arms move inwardly during their downward travel so as to effect a conveying action on the material, guiding it toward the center part of the machine.

The complete head 213 is mounted for rectilinear reverse lateral adjustment on the track or guideway 212 by virtue of a nut holding boss 231 (see Fig. 4) which projects from the rear of the casing 214 through an elongated horizontal slot in the guideway 212 and receives a rotatable screw 232 held against longitudinal movement in bearing boxes 233 and driven by a rotary hydraulic motor 234 through gearing in a gear case 235 which is mounted on the frame 203.

The supplemental mining mechanism of the machine of Figs. 1 and 2, 3, and 4, and Figs. 5 through 8 attacks an area of the mine face sufficient to permit the machine to follow through or into the cut area, but it does not attack this entire working face, entry or tunnel at one It is designed to travel successively the sides of a generally rectilinear path, theheight of which is adjustable andthe width of which is wider than the machine, until the whole working face has been'broken down. The maximum uppermost positior; to which the breaking and chipping head213 may be adjusted is illustratedin dotdash lines in Fig. 2. As clearly illustrated in this figure of the drawings, the combination cutter bars and gathering mechanisms, including chains 200 and 201, preferably extend forwardly beyond the breaker arms 221, 222 whereby they atack solid coal of the mine room face adjacent the mine floor in advanceof the breaker arms 221 and 222 by cutting akerf in it.

It will be seen-that this kerf cut by the chains 200 and 201 relievesor undercuts that coal which is to be acted upon by the lower breaking and chipping arms 221 and 222 and that the breaking and chipping arms 221 and 222 undercut that coal which is to be actedupon by the upper breaker arms 223and 224. Thus all of the'coal to'be acted upon by the breaker arms 221 through224 is relieved.

This machine has the desirable characteristics mentioned in connection with the machine of Figs. 1 through 10 in that it removes coal from the-working face over a lateral dimension greater than the maximum width of the machine so that the machine is free'to move laterally. Likewise, it will remove the coal through adjustable vertical dimensions the minimum of which is greater than" loading mechanism necessarily cuts a bottom kerf and" movementof the machine in the tunnel, room or entry which is being driven. This is a matter of considerable importance in any such machine, for a machine which binds itself in a tunnel, room or entry which it creates soon loses its maneuverability andusefulness because of irregularities 'in the mine vein which the machine must be adaptedto follow, as well as freedom of movement of the working tools which attack the coal which if they become bound either become totally inoperative or require an extenive amount of power not practically available to operate. them.

This

It is, of course, to be understood that the relatively.

short bars 24 and 25 with their breaking and loading chains 41 and 42 shown in the machine of Figs. 1 through 8 may be substituted in the machine shown in Figs. 1 and 2, and 2 and 4, and vice versa, if desired.

As previously mentioned, the breaking and chipping head 213 is carried to follow or travel successively along the sides of a generally rectilinear path, the height of which is adjustable. The head 213 is driven along said rectilinear'path by the motor 210 and motor 234 which are in and form a part of a hydraulic circuit or system that is illustrated diagrammatically in Fig. 9 of the' drawings.

There are four plunger type valves 236, 237, 238 and 239 in this hydraulic system that are operated by movement of the head 213 :as .it approaches and departs from extension by bolts .243 .(see Fig. 2). of the vertical positions of valves 237 and238 theheight 0f the rectangular path of travelofthe head 213 may be of said spool 256.

6 the end of its travel or side A. When valve 236 is opened 'it causes the head .213 to travel .upward along side A ofithe rectangle,thereby releasing the valve 236.

As the head 213 traveling upwardly along side A approaches valve 237, valve :237 is openedand head213 begins to travel to the right along thetopor side B of the rectangle and valve .237 is released. When head 213 traveling to the right along side B approaches "alve 238,

valve 238 is opened and head 213 begins to travel downwardly along side C of the rectangle and in so doing it releases valve 238. When head 213 traveling along side C of the rectangle approaches valve 239, valve 239 is opened and head 213 begins to travel tothe left along side D of the rectangle, thereby releasing valve .239. This cycle of operation is continuous.

Each valve 236 and 239 is rigidly carried by a forwardly extending extension 240 (Figs. .2 and 4) .of one of the brackets 208. Valve 239 is positioned with its spool extending upwardly so that when the supplemental mining mechanism202 approaches its lowermostrposition, as seen in full lines in Fig. 2 of the drawings, its spool willbe operated by an arm or rearwardly extending portion 241 of the boss 231, which follows the same path as does the head 213. Valve 236 ispositioned with its spool extending toward valve 239 so that said arm 241 will operate it as the arm travels laterally.

Each of the valves 237 and'238 are supported for vertical adjustment from the extensions 240 by a slide or bracket member 242 that is clamped adjustably to the By the adjustment determined. It is to be noted that the spool of valve 237 faces valve 236 and that the spool of valve 233 faces valve 237 so that the spools will be operated by movement of-the arm 241.

Referring now particularly to Fig. 9 of the drawings, the hydraulic system includes a tank 250 and a pump which may boom of the pumps 48 that supplies hydraulic fluid under pressure to operate the motors 210 and 234 and "other valves of the system. It also includes a reversing valve 251, an eight-way valve 252, the four control valves 236 through 239, a throttle valve 253, a pressure reducing valve 254 anda high pressure relief valve 255.

The reversing valve 251 is a spool type valve having a spool '256 and a fluid chamber 257 and 258 at each end When fluid under pressure is admitted to either of the chambers 257 or 258, the spool 256 is shifted to the right or left and displaces hydraulic fluid from the other chamber. Eight-way valve 252 is also a spool type valve having a spool 259 and chambers 260 and 261, which are for the same purpose as chambers 257 and 258 of reversing valve 251.

The control valves 236, 237,238 and 239 are mechanically operated spool valves that are normally held in their closed position by spring pressure and in the operation of the machine only two of them are at any given time operated by the arm 241 that travels with the head 213.

Fig. 9 of the drawings illustrates the parts of various of the valves in those positions which they occupy when the head 213 has traveled along the side'of the rectangular path illustrated in Fig. 10 of the drawings in which the valve 236 is held in its open position by the arm 241.

When valve 236 is open, as shown, hydraulic fluid flows from pump discharge pipe 262, pressure reducing valve 254, pipe 263, control valve 236 and pipe 264 to cham her 257 of reversing valve 251, and spool 256 of valve 251 shifts to'the left as shown in the drawings. Hydraulic fluid from chamber 258 is released through pipe 265, control valve 236 and pipes 266 and 267 to the tank 250. Valve 251, upon shifting to the position shown directs hydraulic fluid from pipe 262 through throttle valve 253, pipes 276 and 269, eight-way valve 252, pipe 7 270, motors 210, pipe 271, pipe 272, valve 252 and pipe 273 to tank 250. Motors 210 raise the head 213 along side A of the rectangular path of travel, and control valve 236 closes isolating pipes 264 and 265 thereby locking spool 256 in the position shown.

When control valve 237 is opened by arm 241, hydraulic fluid flows through pipe 263, control valve 237 and pipe 274 to the bottom chamber 260 of eight-way valve 252. Spool 259 of eight-way valve 252 shifts upwardly and forces hydraulic fluid from chamber 261 through pipe 275, control valve 237 and pipe 267 to the tank 250. When the spool 259 of eight-way valve 252 shifts upwardly it closes pipe 269 and opens pipe 276. Hydraulic fluid then flows through pipes 268 and 276, eight-way valve 252, pipe 277, motor 234, pipes 278 and 279, eight-way valve 252 and line 273 to tank 250. Motor 234 then rotates in the proper direction to drive the head 213 to the right along side B of its rectangular path of travel. When head 213 moves to the right, control valve 237 closes and isolates pipes 274 and 275, thereby locking spool 259 in its raised or up position.

When control valve 238 is opened by arm 241, hydraulic fluid flows through pipe 263, control valve 238 and pipe 265 to the left chamber 258 of control valve 251, and spool 256 is shifted to the right. Hydraulic fluid is released from chamber 257 to tank 250 through line 264, control valve 238, lines 280 and 273. When spool 256 moves to the right, hydraulic fluid flows from pump discharge pipe 262 through throttle valve 253, valve 251, pipes 281 and 282, eight-way valve 252, pipe 271, motors 210, pipes 270 and 283, eight-way valve 252 and pipe 273 to tank. Motors 234 then rotate in the proper direction to move head 213 downwardly along side C of its rectilinear path of travel. When cutter head 213 moves downwardly, control valve 238 closes and isolates pipes 264 and 265, thereby locking the spool 256 of reversing valve 251 in its right-hand position.

When control valve 239 is opened by arm 241, bydraulic fluid flows from pipe 263 through control valve 239 and pipe 275 to the top chamber 261 of eight-way valve 252. The spool 259 of valve 252 shifts downwardly to the position shown and hydraulic fluid is released from the bottom chamber 260 to tank 250 through pipes 274, control valve 239 and pipe 284. When spool 259 of eight-way valve 252 is in its lower position, hy-

draulic fluid flows from pump discharge pipe 262 through throttle valve 253, reversing valve 251, pipes 281 and eight-way valve 252 and pipe 278 to motor 234. From motor 234, hydraulic fluid flows through pipes 277 and 235, eight-Way valve 252 and pipe 273 to tank 250. Motor 234 is then driven in the proper direction to move the head 213 to the left along side C of the rectilinear path of travel. When head 213 moves to the left, arm 241 releases control valve 239, and pipes 274 and 275 are isolated. thereby locking the spool 259 of eight-way valve 252 in its lower position. When the cutter head moves to the left it will again open control valve 236, and the above described cycle will be repeated.

The throttle valve 253 functions to maintain pressure in pipe 262 at all times which is at least sufiicient to operate the reversing valve 251 and eight-way valve 252 which are in the low pressure circuit of the hydraulic system. Any of the now very Well known commercially available throttle valves capable of this function may be employed in the system as the valve 252.

Obviously, those skilled in the art may make various changes in the details and arrangement of parts without departing fro mthe spirit and scope of the invention as defined by the claims hereto appeneded, and applicant therefore wishes not-to be restricted to the precise construction herein disclosed.

Having thus described and shown an embodiment of 8 the invention, what it is desired to secure by Letters Patent of the United States is:

1. A coal mining machine including a power driven main frame, mining mechanism including a plurality of spaced rotary coal cutter and breaker arms rotating on spaced parallel forwardly projecting shafts on horizontal axes, mechanism connecting said mining mechanism to said main frame for up and down and lateral movement whereby said mining mechanism may be moved to follow a rectangular path, means for moving said mining mechanism as aforesaid including hydraulic motor means, four hydraulic control valves, a hydraulic circuit including said hydraulic motor means and said valves, said valves operating to change the direction of movement of said mining mechanism at each corner of said rectangular path of movement of said mining mechanism.

2. A coal mining machine including a power driven main frame, mining mechanism including means for cutting and breaking coal from a mine room face, mechanism connecting said mining mechanism to said main frame for up and down and lateral movement whereby said mining mechanism may be moved to follow a rectangular path, means for moving said mining mechanism as aforesaid including hydraulic motor means, four hydraulic control valves, a hydraulic circuit including said hydraulic motor means and said valves, said valves operating to change the direction of movement of said mining mechanism at each corner of said rectangular path of movement of said mining mechanism.

3. A coal mining machine including a power driven main frame, mining mechanism including means for cutting and breaking coal from a mine room face, mechanism connecting said mining mechanism to said main frame for up and down and lateral movement whereby said mining mechanism may be moved to follow a rectangular path, means for moving said mining mechanism as aforesaid including control means operating to change the direction of movement of said mining mechanism at each corner of said rectangular path of'movement of said mining mechanism.

4. A coal mining machine comprising a self-powered mobile frame, mining mechanism including means for cutting and breaking coal from the coal face, mechanism connecting said mining mechanism to said frame for consecutive movement in four different directions each being at an angle to the preceding movement in a plane transverse to said main frame, motor means for moving said mining mechanism in all of said directions in said transverse plane, and a control mechanism for said motor means to determine the travel of said mining mechanism in any one of said directions and for changing its direction of travel.

References Cited in the file of this patent Colmol article in Coal Mining magazine, February 1949, pages 15, 16, 17 and 18. 

